1. Field of the Invention
The present invention relates to an image forming apparatus such as a digital copying machine or a printer, and to a control method for the image forming apparatus.
2. Description of the Related Art
In a conventional digital copying machine, when a copying operation is performed, an original is fed to a scan mechanism and a paper sheet is fed to a print/output mechanism. Thus, the copying operation is executed. In this operation, there is a case where a sheet-reversing section for reversing a paper sheet is provided in front of the output mechanism.
Specifically, in order to successively output copied paper sheets in the order of page numbers, the obverse and reverse sides of a conveyed paper sheet are turned upside down by the sheet-reversing section. In addition, in order to perform double-side printing, a paper sheet having an image on its one side is reversed by the sheet-reversing section and brought to an automatic double-side unit.
However, when a paper sheet is set in a sheet feed cassette of a digital copying machine, for example, when a thick paper sheet is set, the thick sheet (e.g. 209 g sheet), which has a greater thickness (and a greater resiliency) than an ordinary paper sheet, may cause friction with the guide member of the convey path. As a result, large friction noise is produced when the thick paper sheet passes through a guide-shaped R-portion (reversing section).
Besides, in a modern high-speed machine, a sheet feed interval of paper sheets is decreased to increase a copy productivity (CPM: copy per minute). In this case, in order to reverse and output (or discharge) the sheet, the sheet convey speed at the reversing section needs to be increased. To achieve this, a speed acceleration control is executed to accelerate the sheet convey speed at the time of reversing the sheet. Specifically, a paper sheet on which an image is formed is fed at a constant speed until it passes through a fixing device. After a rear end of the sheet comes out of the fixing device, the convey speed is accelerated at a predetermined timing.
In the apparatus where the sheet-reverse section is used to effect both operations for the reversed-sheet output and the sheet reverse conveyance to the automatic double-side unit, the sheet-reversing positions for the respective operations are determined. The sheet-reversing positions are determined by the timing provided by sensors disposed in the convey path. In short, as the convey speed increases, a variation increases in the sheet-reversing position due to an error in timing or a slip of rollers.
In addition, in general, a sheet-reverse position for re-feeding the sheet to the automatic double-side unit is set on the downstream side of a sheet-reverse position for reversing the sheet and outputting the reversed sheet. In a case where an LD sheet with a large length is used, if the sheet-reverse position shifts to the downstream side, a front edge of the sheet abuts upon an end wall of the convey path, resulting in folding of the sheet or noise due to abutment.
On the other hand, if the sheet-reverse position shifts to the upstream side, the reverse conveyance to the automatic double-side unit would begin before the sheet does not completely come out of the convey path. As a result, a jam may occur in the vicinity of the entrance of the automatic double-side unit or noise of abutment may occur in the convey path.
Furthermore, the sheet-reverse position alters due to the convey speed. If there is a variance among machines due to precision of parts, such as a roller diameter, or assembling, the sheet-reverse position would vary. As a result, like the above-mentioned case, such problems as folding of paper, jamming or noise due to abutment may arise.